The book gives an answer of -(pi/3) + k(pi), which I'd think is correct over my equations - I honestly don't know how to get to an answer because the book is horrid at teaching and I'm out of school for a bit so I can't ask the teacher for help.

Sometimes people forget the simple stuff and use the less simple stuff; they may forget to recognize what they have already studied. Usually, the students still UNDERSTAND the simple stuff, but they just forgot that this simple stuff is all that is needed.